My 89 525 driver's side window does not close properly. The window or perhaps the window tracks within the door appear to be loose. This results in the window "jumping" out of the channels in the window frame. Needless to say, this is a pain in the neck...literally.

How difficult is it to track this down? I have the Bentley manual and am somewhat mechanically inclined. Has anyone had this problem and, if so, was the fix relatively easy?